The present invention relates to a method for treating plastic polymers to reduce or remove organic contaminants. More particularly, the present invention relates to a method of treating, by continuous means, a flowable polymer mass with a solvating fluid in an environment at which the solvating fluid is in a supercritical state and is subject to conditions sufficient to preferentially solvate and extract organic, and especially non-volatile, contaminants from the polymer mass.
The plastics industry has recently begun to focus more attention on the use of recycled plastics in the manufacturing of new plastic materials for both industrial chemical and food-grade applications. This has been in response to the public demand for decreasing the amount of waste that we produce (which can lessen our reliance on landfills and waste-to-energy facilities) and for making more efficient use of our resources (e.g., energy).
The plastics industry has recognized that recycled plastics can serve as an economical substitute for virgin materials. The challenge, however, has been to devise appropriate and economical means of processing recycled plastics for use as substitutes for virgin materials for a wide variety of applications.
Although various methods have been developed for processing of recycled plastics, there remains a need to develop commercially efficient and economical means for processing of recycled plastics for use as substitutes for virgin materials, as are normally required for high purity polymeric materials or food-grade applications. Most presently known processes are inadequate for obtaining high purity recycled plastics, for commercial purposes, because such processes are primarily directed to the reduction or removal of volatile or surface contaminants, or are inappropriate to meet industrial demands for high volume and high efficiency processing.
Accordingly, there remains a need to develop processing techniques that remove both volative and non-volatile organic contaminants which can provide high purity polymeric materials from recycled plastic polymer feedstocks for use as substitutes for virgin materials.
In addition, governmental authorities around the world have begun to promulgate regulations for plastic materials to establish basic guidelines for the use of recycled plastics in food-grade applications. The two primary guidelines are as follows: (1) the packaging will not endanger the consumer through product adulteration by migration of material from the package; and (2) the package will not detract from the taste and smell of the food.
The Food and Drug Administration (FDA), the regulating authority in the United States, has established a threshold of regulation for indirect food additives from plastic packaging as 0.5 ppb dietary intake. This threshold level defines the maximum migration from the plastic into the food that the FDA has determined to be an acceptable risk.
In the processing of plastics, this threshold level may be attained in the end use material by reduction or removal of contaminants to this level, or by mixing an appropriate amount of virgin materials with the recycled materials.
The FDA has also devised a test protocol that may be used to determine whether certain recycled plastics meet its threshold of regulation. The FDA has identified the limits for various plastic polymers in conduction with a series of surrogate chemicals that the FDA has deemed to be representative of the estimated 60,000 chemical in commerce.
These surrogates were chosen to represent the various physical and chemical classes of compounds, and cover the categories of polar volatiles, polar non-volatiles, non-polar non-volatiles, non-polar volatiles, and metallics/organometallics. Polar volatiles include chloroform and 1,1,1-trichloro-ethane; polar non-volatiles include diazinon, tetracosane, and benzophenone; non-polar non-volatiles include lindane, squalane, eicosane, and phenyldecane; non-polar volatiles include gasoline and toluene; and organometallics include disodium monomethyl arsonate, zinc stearate and copper II ethyl hexonate.
FDA testing has also provided the threshold limits for various plastic polymers with respect to these chemicals.
Accordingly, it would advantageous to the industry to develop appropriate means to recycle plastics to such purity levels. Various means are known in the art for removal and extraction of impurities and contaminants.
The removal or extraction of impurities and contaminants from a wide variety of plastic polymers using fluids that are at or near supercritical conditions as an extractant or solvating fluid is well known. Extraction of contaminants with supercritical fluids such as carbon dioxide as compared to organic solvents has advantages of lower cost, ease of operation and most importantly eliminates the disposal problems associated with organic solvent waste. However, the processes as known and practical to date generally suffer from inherent disadvantages in that they are either batch processes using one or more extraction vessels such as autoclaves or are slow and/or relatively inefficient in design. The known processes do not result in efficient removal of unwanted contaminants and do not provide a purified polymer in a form that is easily useable without further processing, i.e., without remelting and repelleting.
U.S. Pat. No. 4,563,308 (assignee Stamicarbon) discloses batch removal of impurities from a ethylene-alkene-diene rubber in an autoclave using a supercritical fluid such as carbon dioxide, nitrogen, oxide, nitrogen dioxide, sulfur dioxide, etc. EP Application No. 233,661 (assignee Stamicarbon) discloses the supercritical extraction of impurities from a molten polymer in an extruder in which the exit die functions as a pressure seal to establish a supercritical pressure in the extruder. The supercritical fluid is mixed with the polymer under high pressure in the barrel of the extruder and the impurities become dissolved in the supercritical fluid. The pressure on the mixture is instantaneously released to atmospheric pressure upon exiting the extruder causing vaporization of the impurity containing supercritical fluid from the polymer. This arrangement results in the inability to maintain adequate control over the pressure in the extruder barrel which causes nonuniform flow of the supercritical fluid. This causes erratic flow of the polymer mass in the extruder and produces a nonuniform polymer product, that may be characterized by a foaming of the polymer product or a xe2x80x9cpopcornxe2x80x9d effect. In order to provide a commercially saleable product, remelting and pelleting of the extruded polymer is generally required. Further, the efficiency of contaminant removal is relatively low.
U.S. Pat. No. 5,237,048 (assignee Toyo Engineering) discloses the removal of volatile impurities from a molten polymer with a supercritical fluid. A wide variety of polymers and supercritical fluids are disclosed and the extraction is carried out under high pressure in a countercurrent extraction tower.
U.S. Pat. No. 4,902,780 (assignee Rhone-Poulene Sante) discloses removal of residual monomers from a styrenevinylpyridine copolymer with supercritical carbon dioxide in an autoclave.
U.S. Pat. No. 4,703,105 (assignee Dow) describes a method for treating a reaction mixture of styrene polymerized with an equal amount or more of acrylonitrile and containing free styrene and acrylonitrile monomers with supercritical carbon dioxide or sulfur hexafluoride in a series of fluid extractors.
U.S. Pat. Nos. 5,049,647, 4,764,323 and 5,073,203 (assignee CoBarr) disclose methods for purifying polyethylene terephthalate resin by contacting the resin with an atmosphere containing carbon dioxide under supercritical conditions in an autoclave.
U.S. Pat. No. 5,049,32 (assignee Airco) and U.S. Pat. No. 5,133,913 (assignee Toyo Engineering) disclose the use of supercritical fluids to both remove volatile impurities from a variety of plastic polymers and to function as blowing agent for foaming the resulting polymer. U.S. Pat. No. 5,009,746 discloses an extensive bibliography directed to the use of supercritical fluids to extract impurities from a wide variety of substrates.
A process for reducing the acetaldehyde content in polyethylene terephthalate chips is disclosed in U.S. Pat. No. 4,223,128 to Hallick, et al. The process comprises stabilizing the polyethylene terephthalate by heating it at an elevated temperature in air and maintaining an air to chip ratio at a predetermined value of at least about 0.8 standard cubic foot of air per minute/pound of resin per hour and at a vapor velocity of at least about 0.5 foot per second.
U.S. Pat. No. 5,080,845 (assignee Werner and Pfleidere) discloses the removal of impurities from plastic polymers in two serially connected extruders using supercritical carbon dioxide in the first extruder. In the first extruder, the plastic polymer is contacted with an extraction gas at supercritical pressure to dissolve the impurities. The mixture of the plastic polymer and the carbon dioxide is then transferred through a pressure-relief valve to a second extruder. In the second extruder, the reduced pressure instanteously vaporizes the supercritical carbon dioxide containing dissolved impurities, which is vented from the polymer. The polymer is then subjected to vacuum for removal of any residual gases and extruded as granular product. The reduced pressure in the second extruder operates to promote separation of the carbon dioxide from the polymer as a gas, but will not carry off non-volatiles which will be reabsorbed into the molten polymer.
There are two basic types of impurities which occur with respect to polymers in which the method of the present invention has utility. Some virgin plastics, upon polymerization contains a distribution of species having different molecular weights. The low molecular weight components comprising unreacted short chain monomers, diners etc., collectively referred to as oligomers, must be significantly reduced for many end uses of the fully reacted polymer. The removal of low molecular oligomers eliminates problems of noxious hydrocarbon vapor formation during processing and generally improves the handling and workability of the polymer end product. The oligomers are also preferably removed if the polymer is intended for use in food applications, for example as packaging, to avoid permeation or leaching of the oligomers from the packaging material into the food product.
The second type of impurity or contaminant is ordinarily found in refuse plastic which contain impurities resulting from contact with a material during prior usage. In some instances, these impurities may be toxic or hazardous and desirably are removed in order to avoid having to dispose of the refuse in a hazardous material site. In other instances, to which the present invention is particularly directed, plastic materials obtained from ordinary refuse collection may be processed to low impurity levels such that they may replace all or part of the virgin polymer raw material in the manufacture of second generation articles.
The present invention is particularly useful in the removal of unwanted contaminants from high density polyethylene (HDPE). HDPE is a common resin material for blow molded bottles which are used for the storage of milk, detergents, pesticides and motor oil and a significant amount of HDPE is used in consumer product packaging. Polyethylene""s properties, particularly HDPE, allow it to be reprocessed, i.e., recycled. A significant concern in recycling polyethylene is the trace levels, i.e., less than 200 ppm, of contaminants inherently associated with the recycle HDPE stock that cannot be removed by conventional washing procedures. In many cases, the contaminant, for example, d-limonene, benzene, toluene, permeates into the plastic through absorption or adsorption. Reprocessing of HDPE by conventional remelting in an extruder may result in the generation of volatile contaminant fumes which may be harmful to the environment and to personnel operating the equipment, and which, if not substantially removed from the reprocessed HDPE, will prevent the recycled HDPE from being used in many applications, for example in containers for foods intended for human consumption
The method of the present invention can be used to remove unwanted contaminants from both virgin polymers and from recycled polymers. As more fully set forth herein, the method is readily practiced in the efficient removal of unwanted contaminants. When used to process polymers collected as refuse, the process can-upgrade in the value of the processed material in the marketplace.